Risk of fire is very low, especially if using solder stations with proper holders for when the irons aren't in use.
Regarding builders, we can obviously discuss if anyone wants theirs built instead of doing it themselves, but there's a lot of satisfaction when you're using an amp you crafted yourself!
In terms of matching with various cans: anything of 300 ohm and up is a great match with the Crack. Below 300 ohm you may have success, but it will be all about synergy and whether the impact of the Crack's relatively high output impedance does good things or bad things to the particular headphone. For example, my new FA-011 LEs (thanks again Billy!) sound OK out of the Crack, but they sound awesome from my solid state - all depends on synergy.
Some known and recommended (not by me, but from my research) matches:
- HD600 / 650 / 800
- Beyer 600 ohm (and possibly some 250 ohm models)
There are comments about it pairing well with lower impedance cans like K701/2, K501, even Etys, but I can't comment and wouldn't want to recommend that based on the numbers, but often the results can be contradictory to the numbers (e.g. my HFI-680s were improved by the AK100s high OI)